Boilers for generating steam from water are well known, the steam being used typically for motivating steam engines or steam turbines, for heating, for cooling, for cleaning and sterilizing, and for many other known uses. Oil heaters for providing hot oil as an energy transfer medium are likewise well known. (As used herein, the term “boiler” should be taken to mean boiler or oil heater, and, except where noted, the invention as described for boilers should be understood as being also applicable to oil heaters.) Such boilers are known to be fueled by a variety of energy sources, for example, nuclear decay and hydrocarbon combustion. Some typical hydrocarbon fuel sources are wood, coal, fuel oil, and natural gas.
A particular class of boiler systems employs an injectable hydrocarbon fluid fuel, such as fuel oil or natural gas, which may be readily supplied under pressure to a boiler via a pipeline, and which may be readily metered via a fuel control valve to a burner disposed within the boiler. Fuel oil injection may be assisted by an auxiliary steam injector. Typically, the fuel is injected axially at a first end of a generally cylindrical or rectangular, elongated firing chamber. A high-capacity blower, or air pump, introduces combustion air via an air flow control valve, or damper, into the firing chamber in the region of the injector, and fuel and air flow axially of the firing chamber. Ignition is initiated by an independent pilot light system to produce an elongate burner flame. The air flow typically is divided into at least a primary flow introduced axially of the flame and a secondary flow introduced peripherally of the flame, whereby the rate of burn and shape of flame may be modified. The firing chamber is generally surrounded by, and in contact with, an array of water-conveying boiler tubes continually supplied with water. Heat from combustion is transferred by conduction, convection, and radiation through the walls of the firing chamber and the tubes to heat and ultimately boil the water, producing steam. The steam generated is collected at a boiler drum and is conveyed to points of use via a steam header. The cooled flame gases are exhausted, typically to the atmosphere, via a stack.
In some prior art boiler systems, the fuel control valve and air control valve are linked via either mechanical or electrical means such that the fuel and air flows vary together in an apparently fixed ratio, which ratio is determined experientially to produce an “acceptable” flame. An acceptable flame is one that produces both the required volume of steam and an environmentally acceptable exhaust, without particular regard to the fuel efficiency of the flame in producing the steam. The ratio, however, is not truly fixed, since the actuation functions of a typical valve and damper are not linear.
In some prior art boiler systems, there typically is no means for optimizing various process parameters to produce the most steam for the least fuel. For example, there is no means for systematically optimizing the total air flow or the air-to-fuel ratio: too much air can result in excess heated air in the exhaust, which is wasteful; too little air can result in sub-optimal combustion, coking of the boiler tubes, and hydrocarbon residues in the exhaust. Further, improper primary and secondary air control, as well as improper total air control and fuel control, can result in a) highly localized combustion in relatively short regions along the length of the firing chamber, which combustion thereby under-utilizes a substantial portion of the total heat-exchanging surface area, and b) a chaotic and unstable flame which only partially adheres to the walls of the firing chamber, thereby permitting a substantial portion of the flame to pass through the system without making contact with a heat-transfer surface.
Further, in the prior art, the process controller operates from the beginning at start-up by feedback control from random positions of the control operators, making iterative changes to each input setting as the controller recognizes that the designated process control output parameter value still does not match the setpoint value. The controller has no a priori “knowledge” of what the ultimately correct settings will be, and thus such settings are essentially experimentally re-determined every time the process is started up. Further, the controller has no predetermined means for optimizing the overall process by mutually optimizing the setting of each input operator. Thus, although the output value eventually matches the setpoint, by definition placing the process in control, it is highly unlikely that the combination of settings which is optimum for fuel efficiency has been determined. For example, in firing a steam boiler to achieve a setpoint value for steam flow and/or steam pressure, there may be literally thousands of combinations of settings and conditions for fuel flow, primary air flow, secondary air flow, trim air flow, total air flow, and flue gas recirculation flow which will cause the system to provide proper steam flow at the proper pressure. However, only one or at most a very few of such combinations include the minimum fuel flow. The prior art controller has no means of determining what that combination is, and therefore has no means for moving the process towards it.
Further, some prior art boiler control schemes utilize proportional-integral-differential (PID) logic for controlling fuel and/or air flow to the burner, which can result in substantial overshoot and cycling of the process during startup and at other points of significant process instability.
Further, some prior art boiler control systems are extremely difficult, time-consuming, and costly to trouble-shoot to determine the cause of a process failure.
What is needed is a method and apparatus for controlling the generation of steam by a fluid-fueled steam boiler system, wherein at least the flow of fuel, the flow of primary air, and the flow of secondary air are independently and optimally controlled to generate a given flow of steam at a given manifold pressure and a stack exhaust meeting environmental quality standards, while using a minimum flow rate of fuel.
What is further needed is a control logic that brings a steam boiler system into process control rapidly and minimizes process overshoot and cycling at start-up of the process.
What is further needed is a steam boiler process control system that can identify immediately causes of process failures.
It is a principal object of the present invention to minimize the fuel cost of operating a steam boiler system.
It is a further object of the present invention to increase the reliability and therefore extend the runtime of a steam boiler system.
It is a still further object of the present invention to provide easy trouble-shooting of process anomalies and failures in operation of a steam boiler system.
It is a still further object of the invention to bring a steam boiler system into steady-state control rapidly and with minimum process cycling.